As a device attached to a fuel suction opening disposed in a fuel tank for, e.g. an automobile and a motorcycle to filter fuel sucked through the suction opening by a fuel pump, there has been known a filter device which includes a bag-shaped member formed of a filter material (see JP-A-2000-246026). In such a type of filter device, it is necessary to sufficiently separate water from oil. Even if it is possible to reliably separate water from oil in a normal situation, it is likely that the pressure generated by the volume expansion of frozen water in water freeze presses water into a filter in, e.g. cold climates, with the result that the water is frozen in the filter. Even if water is frozen as stated above, it has been desired to prevent water from coming into the filter device.